Much has been in the news recently about an early grapevine bud break in 2018 and concerns regarding increased vulnerability to frost. Bud break is the first major biological process by the grapevine annually as it shakes off winter dormancy. From buds formed in the prior growing season, new small vine shoots start to emerge that will eventually become the current harvest. This event occurs when the main vine senses that temperatures are sufficiently warm and the soil contains enough moisture to safely begin the growth cycle. Of course, one of the concerns is that the vine can be fooled by a stretch of warm weather at the tail end of winter when future cold frost can still occur with devastating impact on the fragile shoots.
Early February 2018 in Napa and Sonoma has been unusually warm with resulting reports that the first bud break has been observed in vines of the southern Carneros region. Typically these vines will be white varieties Chardonnay or Sauvignon Blanc that are consistently the first to emerge from winter hibernation. Indications are that this bud break is about two weeks earlier than 2017, although last year was more of anomaly in recent years with a trend toward consistently earlier shoot emergence. Bud break in 2016 was close to what we are seeing in 2018, and 2015 was one of the earliest on record.
As noted above, the largest concern with earlier timing is the possibility of frost. Such is the case this year where our warm early February has been followed by record low evening temperatures later in the month. Estimates are that less than five percent of the total vines have already experienced bud break so the danger is not widespread, but certainly a higher risk for those earlier emerging varieties. Vineyard managers with shoots at risk have scrambled to deploy frost avoidance measures such as deploying fans that look like wind turbines to stir the air moderating the worst effects.
One of the interesting facts is that once bud break begins, subsequent weather conditions will have only minor impact on the actual maturation timing of the grapes. Much like salmon that have an innate sense memory of the home they will return to over large distance in order to spawn, grapes have a genetically encoded time schedule for ripening. This “clock” is different between grape varieties but is largely consistent within the same grape family no matter where it is planted in the world. The timing may vary by a few days for the same grapes but the deviation will not be large. Therefore, an earlier bud break will automatically infer that the timeframe for maturation will be sooner. This generally also correlates to an earlier harvest, although the human decision on picking grapes has subjective elements dictated by the vintner. They may choose to actually pick slightly earlier or later depending on the specific character they are seeking to express in the final wine.
Although, weather after bud break has minimal impact on timing, it does have a major influence on the survival and taste profile of the grape. Rain and hail occurring at the wrong time can virtually destroy the crop of a vineyard by splitting the grapes or knocking them off the vine. Intense sun exposure can actually leave the grapes sunburned without effective leaf or “canopy” management that is essentially the grape version of an umbrella at the beach.
Additionally, a higher amount of heat and sunshine will have the effect of creating more sugars in the grape. It is for this reason that the same varieties of grapes in Europe and America have ripening periods, which are very consistent, but the sugars present at harvest are lower in European vineyards than those in more coastal California. This leads to California wines usually being higher alcohol than European vintages. The changing heat pattern is creating hotter summers in Europe meaning that the alcohol percentages for these wines is moving up, while the somewhat cooler summers in Sonoma mean that alcohol percentages are declining slightly. Therefore, the differences between European and Sonoma/Napa wines are being moderated as a consequence of global climate change. This effect can be seen even much closer to home in California where grapes grown more in the interior regions like Lodi and the Sierra Foothills will have harvest timeframes very similar to Sonoma. However, the heat in these inland areas means their sugars will be at 29-30 Brix while in Sonoma they are at 26 Brix, meaning differences in both alcohol content and the character of the finished wine from these regions.